1. Field of the Invention
The present invention relates generally to optical pickup devices and particularly to optical pickup devices having an objective lens holder with a pair of tracking coils and an objective lens fixed thereto, and a pair of magnets each generating a magnetic field, and the tracking coil is exposed to the magnetic field, while the coil has an electric current passed therethrough to drive the objective lens in a tracking direction.
2. Description of the Background Art
An optical pickup device drives an objective lens holder having an objective lens fixed thereto, in tracking and focusing directions to positionally control the objective lens. The objective lens holder has tracking and focusing coils fixed thereto and is penetrated by a single axis of rotation. Furthermore, around the objective lens holder, a pair of magnets is arranged.
The magnets generate magnetic fields and when the focusing coil is exposed to the magnetic fields and also has an electric current passing therethrough a force is generated according to Fleming's Left-Hand Rule to drive the objective lens holder together with the objective lens along the axis of rotation (i.e., in the focusing direction). Furthermore, when the tracking coil is exposed to a magnetic field generated by the magnet and also has an electric current passing therethrough, a force is generated according to Fleming's Left-Hand Rule to drive the objective lens holder together with the objective lens in a direction of rotation around the axis of rotation (i.e., in the tracking direction).
If a pair of racking coils are asymmetric with respect to an axis of rotation then when the objective lens holder rotates, a magnetically asymmetric state can be provided. To address this, Japanese Patent Laying-Open No. 2002-50060 proposes to arrange the pair of tracking coils at a position in point symmetry with respect to the axis of rotation.
Conventionally when such a point symmetrical arrangement is adopted a pair of tracking coils is often arranged such that when the objective lens is not displaced in the tracking direction the pair of tracking coils is located on a centerline passing through a pair of magnets at their respective centers, since in such arrangement when the objective lens is not displaced in the tracking direction the tracking coils are situated on the magnets' centerline, at which a magnetic field is maximized in intensity, and the objective lens holder can be driven by a maximum force.
FIG. 15 is a cross section showing a conventional arrangement of tracking coils. As shown in the figure, a centerline 19 passes through magnets 14a and 14b at their respective centers. Tracking coils 45a, 45b are air core coils rectangular in geometry. When the objective lens is not displaced in a tracking direction, i.e., when the objective lens holder does not rotate, tracking coil 45a has a vertical portion 47a on centerline 19 and tracking coil 45b has a vertical portion 47b on centerline 19.
Such arrangement as above, however, is disadvantageous as follows: when the objective lens is not displaced in the tracking direction, the objective lens holder is driven by a large force. When the objective lens is displaced in the tracking direction, however, the holder is driven by a reduced force.
FIG. 16 shows a relationship between lens shift amount and tracking sensitivity as conventional. A “lens shift amount” refers to an angle of rotation of an objective lens holder 16 around an axis of rotation 18. A lens shift amount of “0” indicates that objective lens 11 is not displaced in the tracking direction, i.e., that objective lens holder 16 is not rotating. “Tracking sensitivity” refers to a force that drives objective lens holder 16 in the tracking direction. As shown in the figure, when objective lens holder 16 rotates, the tracking sensitivity decreases. This is attributed to the following ground:
When objective lens holder 16 rotates in a positive direction or a negative direction, tracking coil 45a has vertical portion 47a moving away from centerline 19, which is exposed to an intense magnetic field, and thus experiencing a reduced force, and tracking coil 45b has vertical portion 47b moving away from centerline 19, which is exposed to an intense magnetic field, and thus experiencing a reduced force. Thus when objective lens holder 16 rotates in the positive or negative direction objective lens holder 16 is in its entirety driven by a reduced force.
Such a variation in the tracking sensitivity is not limited to the arrangement allowing a pair of tracking coils to be located on a centerline when objective lens 11 is not displaced in a tracking direction. Such variation generally occurs for arrangements having a pair of tracking coils arranged in point symmetry.
Such variation in the tracking sensitivity impairs the optical pickup device's tracking performance. In particular, if a seek operation is performed by tracking of an objective lens, the lens must significantly be displaced in the tracking direction. Increased displacements in the tracking direction, however, result in reduced driving forces, and the lens's position can not be matched to a targeted track's position.